This application is the U.S. national phase of PCT/AT98/00034 filed Feb. 18, 1998, which claims priority from the Austrian application A 337/7 filed Feb. 27, 1997.
The invention relates to a method of recovering a purified von Willebrand factor (vWF) by means of cation exchange chromatography.
In plasma, vWF circulates at a concentration of from 5-10 mg/l, partly in the form of a non-covalent complex with factor VIII. vWF is a glycoprotein which is formed in various cells of the human body and later is liberated into the circulation. At this, a vWF dimer (primary vWF dimer) having-a molecular weight of approximately 450000 Da is synthesized in the cells, starting from a polypeptide chain having a molecular weight of approximately 225000 Da (vWF monomer) by forming several sulfur bonds. From the vWF dimers, further polymers of vWF with ever increasing molecular weights, up to approximately 20000000 Da, are in turn formed by forming links via sulfur bonds.
One important criterion for characterizing vWF is the multimer-structure analysis by agarose electrophoresis. It is assumed that particularly the high-molecular vWF polymers are of essential importance in the coagulation of blood. The functional activity of vWF usually is determined by the ristocetin-cofactor-activity (vWF:RistCoF). The ratio between activity and vWF antigen concentration (vWF:Ag) is determined as a characteristic for the purity and specific efficacy of vWF. The specific activity of a preparation increases with an increasing ratio of vWF:RistCoF to vWF:Ag.
vWF assumes an important function within hemostasis. It circulates in plasma partly as a complex with factor VIII. which aids the coagulation of blood as a cofactor. Factor VIII is stabilized by complex formation with vWF and protected from proteolytic degradation thereby. A further object of vWF is its participation in thrombocyte aggregation which makes an important contribution to primary hemostasis. At this, vWF binds to the glycoproteins Ib and IIb/IIIa of the surface receptors of the thrombocytes and thus cross-links the thrombocytes to a thrombocyte aggregate. What is furthermore important for primary hemostasis is the affinity of vWF to collagen, a component of the extracellular matrix which, in intact vessels, does not have a direct contact with blood, since it is shielded from the blood flow by a monolayer of endothelial cells. When blood vessels are injured, a local detachment of the endothelial cell layer occurs at the site of lesion, resulting in a direct exposure of the components of the extracellular matrix to the blood. By its affinity to collagen, vWF is capable of fixing the forming thrombocyte aggregate in the damaged vessel region to the exposed subendothelium. This results in a first, labile wound closure which will be further strengthened by the subsequent blood coagulation.
von Willebrand syndrome is characterized by a deficiency of a functional von Willebrand factor or by an abnormal spectrum in the multimer composition of the von Willebrand factor. On account of a deficient stabilization of factor VIII, patients afflicted with von Willebrand syndrome may develop a factor VIII deficiency in spite of the fact that usually the synthesis rate of factor VIII is normal, such factor VIII deficiency being a consequence of the greatly reduced plasma half-life of this coagulation factor. Therefore, patients suffering from von Willebrand syndrome may exhibit symptoms similar to those of hemophilia A patients (phenotypic hemophilia). The lack of functionally active vWF may also cause dysfunctions of thrombocyte aggregation and adhesion in patients afflicted with von Willebrand syndrome, which may lead to defects in primary hemostasis. On account of dysfunctions of these vWF-mediated procedures, patients afflicted with von Willebrand syndrome exhibit increased bleeding times.
For the treatment of von Willebrand syndrome, thus vWF preparations must be administered which balance out the lack of functionally active vWF. To this end, preparations may be used which are also used in the therapy of hemophilia A, such as cryoprecipitate or the factor VIII concentrates prepared therefrom, which contain complexes of factor VIII and vWF. However, for the treatment of hemophilia A, better purified factor VIII:C concentrates are always used which either do not contain vWF or contain merely traces thereof. Since supplementing patients afflicted with von Willebrand syndrome with factor VIII is not necessary, and since the factor VIII application harbours the risk of inducing inhibitory factor VIII antibodies in the patient, a vWF preparation as free as possible from contaminating factor VIII would be very desirable for the treatment of von Willebrand syndrome. Therefore, there is a demand for pure and virus-safe von Willebrand factor preparations having a high specific activity.
In the literature, various methods of purifying and recovering vWF have been disclosed.
EP 0 503 991 describes the purification of vWF from human cryoprecipitate by three successive chromatographic steps: 1) anion exchange chromatography on TSK-DEAE Fractogel, and elution of vWF by means of 0.15 M NaCl; 2) another anion exchange chromatography on TSK-DEAE Fractogel, and elution of vWF by means of 0.17 M NaCl; and 3) affinity chromatography on gelatin sepharose to separate the contaminating fibrinogen. There, amino acid- and calcium ion-containing buffers were used.
WO 89/12065 describes the separation of plasmatic vWF from factor VIII and further proteins by binding the proteins to an anion exchanger and step-wise elution by increasing the salt concentration. The vWF-containing fraction was chromatographed for a second time via an anion exchanger and recovered as a concentrate.
EP 0 469 985 discloses the purification of plasmatic vWF from cryoprecipitate, wherein factor VIII is selectively bound to an anion exchanger in a first step at a salt concentration of 250 mM, while vWF remains in the supernatant. After lowering the salt concentration of the vWF-containing supernatant to a salt concentration of between 100 mM and 150 mM, vWF is bound to a second anion exchanger and eluted at pH 6.6 with 300-350 mM NaCl. There, vWF having an activity of at least 50 U/mg is recovered, which contains a portion of factor VIII of  less than 2%.
DE 39 04 354 describes the recovery of plasmatic vWF from cryoprecipitate, and the separation of vWF from factor VIII by selective adsorption of factor VIII on an anion exchanger, while vWF remains in solution. There, a solution containing 160 U/ml of vWF is recovered.
U.S. Pat. No. 5,006,642 describes the recovery of vWF from a solution of vWF and chaotropic agent, incurred as a by-product according to U.S. Pat. No. 4,361,509, by dialysis against a suitable buffer or by desalting the solution by means of a further chromatographic step.
EP 0 383 234 describes the production of a vWF concentrate by means of anion exchange chromatography, wherein a factor VIII/vWF-complex contained in a solution is dissociated by the addition of a calcium and amino-acid-containing buffer, and a vWF concentrate is recovered.
WO 96/10584 describes a method of recovering highly purified recombinant vWF by means of combined anion exchange/heparin affinity chromatography, and EP 0 705 846 describes the separation of high and low molecular fractions of recombinant vWF by means of heparin affinity chromatography.
To recover a purified vWF preparation having high specific activity, it has been necessary so far to combine several chromatographic steps. In particular, the production of preparations particularly containing high-molecular vWF multimers has so far been possible only via a heparin affinity chromatography. Heparin, however, is a relatively expensive chromatographic material.
It is the object of the present invention to provide a method of recovering purified vWF having an improved specific activity, which method is suitable for a large technical application on an industrial scale. The method should be usable for the purification of both recombinant and plasmatic vWF.
According to the invention, this object is achieved in that a method of recovering vWF is provided, in which vWF at a low salt concentration is bound to a cation exchanger, and vWF consisting particularly of high-molecular vWF multimers having a high specific activity, is recovered by step-wise fractionated elution. The recovery and enrichment of vWF having an improved activity and stability is particularly effected in that by increasing the salt concentration step-wise, at first fractions containing low-molecular vWF multimers, inactive degradation products and unspecific accompanying proteins are separated at a medium salt concentration, and fractions containing high-molecular vWF multimers having a high specific activity are recovered at a higher salt concentration.
On account of its acidic isoelectric point (IEP=5.5 to 6) and its negative net charge resulting therefrom, vWF usually is purified in a weakly acidic to basic environment via positively charged anion exchangers. Thus, on account of the methods of purifying vWF by means of positively charged anion exchangers described so far, vWF, at a pH lying above the IEP of vWF and at a low salt concentration, could not be expected to bind to a negatively charged gel matrix of a cation exchanger and to be selectively elutable therefrom by raising the salt concentration. Neither could it be expected that by a step-wise elution at a salt concentration of xe2x89xa7300 mM, vWF consisting particularly of high-molecular vWF multimers would be obtained.
It has been found within the scope of the present invention that with the method according to the invention, departing from an impure biological material, purified fractions are obtained which are substantially free from contaminating nucleic acids. Thereby, in addition to the unspecific accompanying proteins, also nucleic acids are removed from protein preparations by this method. This effect cannot be performed with conventional methods by means of anion exchangers, since nucleic acids, on account of their negative charge, bind to the anion exchanger, detach from the anion exchanger again by increasing the salt conqgntration, and get into the eluate.
When purifying vWF, particular attention must be paid that, on account of the size of vWF ranging from 500 000 to several millions, only such carrier-materials which do not impede the diffusion and distribution of the vWF molecule in the carrier materials used will result in good purification and good yields. When carrying out the method according to the invention of purifying vWF having a high specific activity by means of cation exchanger, a gel matrix is used which has not only a high loading capacity, is robust to handle and has a clear elution profile, but which also can be used economically on an industrial scale. Thus, the method according to the invention is particularly interesting for the recovery of purified vWF on a large technical scale.
Every known cation exchanger can be used for carrying out this method, cation exchangers having a sulfopropyl- or carboxymethyl-group conjugated carrier being preferred. SP-Sepharose(copyright) Fast Flow and CM-Sepharose(copyright) Fast Flow (Pharmacia), Fractogel(copyright) EMD-SO3 and Fractogel(copyright) EMD COOH (Merck), Poros(copyright) 10 SP and Poros(copyright) 10S (Perseptive Biosystems) and Toyopearl(trademark) SP 550 C and Toyopearl(trademark) CM-650 (M) (TosoHaas) have, e.g., proved to be well suitable.
A large-porous gel having tentacle structure of the type of Fractogel(copyright) EMD-SO3 and Fractogel(copyright) EMD COOH (Merck) has proved particularly suitable for the recovery of purified vWF.
The adsorption of vWF on the cation exchanger is preferably effected at a salt concentration in the buffer of xe2x89xa6250 mM. Preferred adsorption buffers thus have a salt concentration of from 50 to 250 mM, in particular in the range of 150 mM-250 mM (e.g. 150 mM). By a step-wise raising of the salt concentration in the buffer, vWF substantially consisting of high-molecular vWF multimers can be eluted selectively at a salt concentration of xe2x89xa7300 mM. Low-molecular vWF multimers and proteolytic vWF degradation products which are contained in the vWF-containing solution and which have a low specific activity in terms of vWF activity, in particular in terms of ristocetin-cofactor activity, which have a collagen binding activity and which have a specific platelet agglutination activity, are eluted from the cation exchanger at a salt concentration of between xe2x89xa7250 mM and xe2x89xa6300 mM, preferably at 300 mM, and separated.
Adsorption and desorption of vWF may be effected in a buffer containing a mono- or bivalent metal ion as salt, NaCl being preferably used as the salt.
In the method according to the invention, as the buffer system for eluting the proteins bound to the cation exchanger, preferably a buffer solution comprised of buffer substances, in particular glycine, phosphate buffer or citrated buffer, and salt are used. There, the buffer used preferably does not contain any Ca ions.
The elution buffer may have a pH ranging between 5.0 and 8.5, preferably between 6.0 and 8.0.
The method according to the invention may be carried out as a batch method or as a column chromatography.
The optimal parameters, such as salt concentration, pH and temperature, for carrying out the method according to the invention are, however, each dependent on the cation exchanger material used. Optimization of the conditions disclosed within the scope of the present invention for carrying out the method for each individually used cation exchanger type is, however, within the general knowledge of a skilled artisan.
In particular, by means of the method according to the invention a vWF is recovered and enriched, which particularly consists of high-molecular vWF multimers. Low-molecular vWF multimers and vWF fragments having a low specific platelet agglutinating activity are separated selectively, so that fractions particularly containing high-molecular vWF multimers having a high activity and specificity are obtained.
The recovered vWF fraction(s) is (are) substantially free from low-molecular vWF multimers, vWF fragments with a low specific activity, factor VIII complex, factor VIII:C, non-specific accompanying proteins and contaminating nucleic acids.
Any vWF-containing solution may be used as the starting material for recovering purified vWF by means of the method according to the invention. Starting materials are in particular biological materials, such as plasma, a plasma fraction, cryoprecipitate or a supernatant or an extract of a recombinant cell culture.
vWF-containing solutions may, however, also be enriched protein solutions which have been pre-purified by a preceding purification step, e.g. via gel filtration, anion exchange chromatography, affinity chromatography or a combination thereof. By these preceding procedures it is particularly achieved that vWF is enriched and non-specific accompanying proteins, in particular factor VIII or factor VIII-complex, are selectively separated.
According to a particular embodiment of the method of the invention, a vWF-containing fraction enriched via an anion exchanger is used as the starting solution.
By means of anion exchange chromatography, vWF may, depending on the manner of carrying out the anion exchange chromatography, either pass the anion exchanger freely as a non-bound material, or it may adsorb thereto. Thus, e.g., vWF is recovered from a plasma fraction and enriched in that both vWF and factor VIII/vWF-complex bind to an anion exchanger at a low ionic strength and salt concentration in a weakly acidic environment. vWF is then selectively eluted from the anion exchanger at a medium salt concentration of from 150 mM to 250 mM, while factor VIII-complex and free-non-complexed factor VIII desorb only at a high salt concentration of  greater than 300 mM.
An enriched vWF fraction can also be obtained in that a vWF-containing solution at a medium salt concentration between 100 mM and 200 mM is treated with an anion exchanger, wherein factor VIII-complex binds to the anion exchanger, while vWF remains in solution. Bound factor VIII-complex may subsequently be recovered from the anion exchanger by increasing the salt concentration.
According to a particular embodiment, vWF present in an enriched solution with a salt concentration of xe2x89xa6250 mM is recovered directly from the effluent or eluate or is recovered as the supernatant, respectively (in the batch method) and, optionally is bound to the cation exchanger without changing the ionic strength or the salt concentration. The salt concentration may, however, be lowered by diluting, if necessary.
This embodiment has the particular advantage that a simple combination of anion/cation exchange chromatography is possible, without requiring complex re-buffering, dialysis or the like of the enriched proteins.
Thus, an enriched vWF-containing fraction can be obtained by a first chromatographic step, and a purification and separation of high-molecular and low-molecular vWF fractions can be achieved by a subsequent cation exchange chromatography. Yet also other combinations, such as, e.g., affinity/cation exchange chromatography, anion exchange/affinity/cation exchange chromatography are possible to attain a further enrichment and a selective recovery of vWF having a high specific activity.
By means of the above-described method according to the invention, vWF having a high specific activity is at least 80fold enriched from an impure vWF-containing material.
Since, in principle, any biological material may be contaminated with infectious pathogens, the vWF-containing fraction obtained is treated for an inactivation or depletion of viruses so as to produce a virus-safe preparation. To this end, all the methods known from the prior art, such as chemical/physical methods, inactivation by combination of a photoactive substance and light, or depletion by filtration may be used. In particular, a heat treatment in solution or in the solid state, respectively, which reliably can inactivate both lipid-enveloped and non-lipid-enveloped viruses is suitable for an inactivation of viruses. The virus depletion preferably is effected by means of a filtration over nanofilters.
According to a further aspect, the present invention provides a preparation containing purified vWF having a high speficic activity, consisting particularly of high-molecular vWF multimers, obtainable from a vWF-containing solution by cation exchange chromatography. vWF having a high specific activity is enriched starting from a starting material containing, i.a., vWF of low purity and low specific activity, and accompanying proteins, in particular factor VIII or factor VIII complex containing low-molecular vWF multimers, are selectively separated. Thereby, in particular, a preparation containing purified vWF which particularly consists of high-molecular vWF multimers and which substantially is free from low-molecular vWF multimers and vWF degradation products, is recovered.
In particular, the preparation according to the invention has a specific platelet agglutinating activity of vWF of at least 65 U/mg protein, and a specific collagen-binding activity of at least 65 U/mg protein. Likewise, the preparation is characterized in that it is substantially free from factor VIII and has a factor VIII-content of  less than 0.1%, based on the ratio of activity of vWF to factor VIII.
A further criterion for the purity and the low infectiousness of a product is also the absence of contaminating nucleic acids. The preparation according to the invention thus is substantially free from nucleic acids. xe2x80x9cSubstantiallyxe2x80x9d here means that the content of nucleic acids is xe2x89xa60.7, based on the ratio 260/280 nm. The nucleic acid may, however, also be quantitated according to a method, e.g. as has been described in EP 0 714 987 and EP 0 714 988.
When recovering and producing the preparation according to the invention with plasmatic vWF, yet also with recombinant vWF, as the starting material, optionally a virus depletion/or inactivation method is carried out, as has been described above, to remove infectious particles, a virus inactivation and/or a virus depletion in principle being possible before or after each purification step, starting from the starting material up to the pharmaceutical preparation produced. Thus the preparation according to the invention will be virus-safe in any event.
According to a preferred embodiment, the preparation according to the invention is present in storage-stable form. The preparation containing purified vWF with a high specific activity may be provided as a ready solution, lyophilisate, or in the deep-frozen state. On account of its purity, the preparation is particularly stable. It has been shown that the preparation according to the invention is stable for at least 6 months at xe2x88x9220xc2x0 C., in solution for at least 4 weeks at 4xc2x0 C., and as a lyophilisate for at least 1 year. It has been shown that within each respective period of time, the vWF activity is reduced by 10% at the most, and the multimer pattern of the vWF multimers does not show any substantial change.
The formulation of the preparation according to the invention may be effected in a known and common manner. The purified vWF contained in the preparation of the invention, is mixed with a buffer containing salts, such as NaCl, trisodium citrate dihydrate and/or CaCl2, and amino acids, such as glycine and lysine, at a pH ranging from 6 to 8, and formulated into a pharmaceutical preparation.
The preparation may be used for producing a medicament for treating patients suffering from phenotypic hemophilia and vWD.